ipv6_stub: add ipv6_route_input stub/proxy.

Proxy ip6_route_input via ipv6_stub, for later use by lwt bpf ip encap
(see the next patch in the patchset).
Signed-off-by: Peter Oskolkov <posk@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/net/addrconf.h

@@ -248,6 +248,7 @@ struct ipv6_stub {
 				 const struct in6_addr *addr);
 	int (*ipv6_dst_lookup)(struct net *net, struct sock *sk,
 			       struct dst_entry **dst, struct flowi6 *fl6);
+	int (*ipv6_route_input)(struct sk_buff *skb);
 
 	struct fib6_table *(*fib6_get_table)(struct net *net, u32 id);
 	struct fib6_info *(*fib6_lookup)(struct net *net, int oif,

net/ipv6/addrconf_core.c

@@ -134,6 +134,11 @@ static int eafnosupport_ipv6_dst_lookup(struct net *net, struct sock *u1,
 	return -EAFNOSUPPORT;
 }
 
+static int eafnosupport_ipv6_route_input(struct sk_buff *skb)
+{
+	return -EAFNOSUPPORT;
+}
+
 static struct fib6_table *eafnosupport_fib6_get_table(struct net *net, u32 id)
 {
 	return NULL;
@@ -170,6 +175,7 @@ eafnosupport_ip6_mtu_from_fib6(struct fib6_info *f6i, struct in6_addr *daddr,
 
 const struct ipv6_stub *ipv6_stub __read_mostly = &(struct ipv6_stub) {
 	.ipv6_dst_lookup   = eafnosupport_ipv6_dst_lookup,
+	.ipv6_route_input  = eafnosupport_ipv6_route_input,
 	.fib6_get_table    = eafnosupport_fib6_get_table,
 	.fib6_table_lookup = eafnosupport_fib6_table_lookup,
 	.fib6_lookup       = eafnosupport_fib6_lookup,

net/ipv6/af_inet6.c

@@ -900,10 +900,17 @@ static struct pernet_operations inet6_net_ops = {
 	.exit = inet6_net_exit,
 };
 
+static int ipv6_route_input(struct sk_buff *skb)
+{
+	ip6_route_input(skb);
+	return skb_dst(skb)->error;
+}
+
 static const struct ipv6_stub ipv6_stub_impl = {
 	.ipv6_sock_mc_join = ipv6_sock_mc_join,
 	.ipv6_sock_mc_drop = ipv6_sock_mc_drop,
 	.ipv6_dst_lookup   = ip6_dst_lookup,
+	.ipv6_route_input  = ipv6_route_input,
 	.fib6_get_table	   = fib6_get_table,
 	.fib6_table_lookup = fib6_table_lookup,
 	.fib6_lookup       = fib6_lookup,